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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

174

is considered the most exact, and the rapid method of weighing the. ammo-
nium phosphomolybdate directly, with references to the, modifications
necessitated in both methods by the presence of arsenic, vanadium and

tungsten.1

1. Gravimetric   Determination   of   Phosphorus   in   absence  of

Arsenic, Vanadium and Tim&sten.

(rt) FINKKNKK'S MKTIIOD. Reagents : (i) Aimnoniiim molybdate solu-
tion, prepared by dissolving Ho grams of powdered ammonium molybdatc
in a, mixture, of (>.|,o c.c. of water and ido c.c. of ,jo% ammonia. (1) ...-.; 0-925)
and pouring this solution into a cold mixture of 9(10 e.e. of jo% nitric acid
(1) i-iK) and .J.|o e.e. of water. This reagent should be left, at rest for

some days in the. dark,

(a) Washing liquid, prepared by dissolving 151" grams of ammonium
nitrate in water, adding 10 c.c. of cone, nitric, acid and diluting to i litre.

Procedure. 5 grams of slightly phosphoric iron or steel, or 1-2 grams
of cast-iron, or 0-5-1 gram of material very rich in phosphorus, are treated
in a. porcelain dish with nitric acid of 1) [: (about, i.i e.e. JUT gram of
metal), the liquid being evaporated to drvness and the residue ignited and
treated with hydrochloric acid and the silica removed and determine if
required as in the estimation of silicon by the nitric acid method (wop.

172).

The hydrochloric acid solution is evaporated to a syrup in a. '450 c.c.
beaker, this allowed to cool (no basic iron salt should separate on cooling)
and 50-100 c.c. of the molybdie reagent (according to the amount, of phos-
phorus present) stirred in by means of a md to facilitate the separation
of the precipitate. After a stand of about half an hour in the cold, chemi-
cally pun1 solid ammonium nitrate is added to the extent of about 25% of
the liquid, the salt being dissolved by stirring,

After 18 ",!,| hours at the ordinary temperature, the clear supernatant
liquid is filtered by deeantation, the filtrate being collected in a jou c.c,
conical tlask, the beaker ami precipitate washed with the washing
liquid mentioned above until the latter no longer gives the reaction for iron,
and the precipitate then tianslerred to the tilter."

A few c.c, of hot dilute ammonia solution are then poured into the.
beaker to dissolve the small quantity of adherent precipitate and, a porcelain
crucible of jo 45 e.e, capacity 'tared with the lid having been placed
under the funnel, the ammoniacal solution is poured carefully mi to the
filter, the. precipitate rapidly dissolving. The beaker and filter are washed
with faintly ammoniaeal water and the solution evaporated on a, water-
bath to expel the ammonia, the volume being reduced to 8 10 c.c.; 4-5
drops of cone, nitric arid are then added to bring about the re formation
of ammonium phosphomolybdate and the solution evaporated to dryness.

The crucible, is next limited in an ordinary air-oven or, better, a Finkeuer

1 For the determination of phosphorus in products vtit.it i,u k-fl by acids, KM 1'erro-

silicon.

" Wluui the prwipHuU* is transfcrml to the fillm-, it is wt-11 to ri-plaa1 the 300 c.c.
fluHk by a smaller one in order that less liquid will n'qum- iilttnin^ hhnuld d"' filtrate
pass through turbid. taken.ing